Method for Processing Encrypted Data

1. A computer-implemented method for processing a first ciphertext, wherein the first ciphertext is data encrypted with a first key and not encrypted with a second key, wherein the method is implemented using a block cipher and comprises: receiving a swap key, wherein the swap key is the product of an exclusive OR operation performed on the first key and the second key; receiving the first ciphertext, wherein the first ciphertext comprises a plurality of blocks; and performing an exclusive OR operation on the swap key and the first ciphertext to produce second ciphertext, such that the second ciphertext is the data encrypted with the second key and not encrypted with the first key.

2. The method according to claim 1, wherein a mode of operation of the block cipher is: counter mode; or Galois/counter mode; or output feedback mode; or cipher feedback mode.

3. The method according to claim 2, wherein the mode of operation is counter mode, and wherein, for a first block of the first ciphertext, the method further comprises: concatenating a first initialisation vector and a first counter value to produce a first concatenated vector; concatenating a second initialisation vector and the first counter value to produce a second concatenated vector; producing first encryption data by encrypting the first concatenated vector with the first key; producing second encryption data by encrypting the second concatenated vector with the second key; and performing an exclusive OR operation on the first and second encryption data to produce a first swap key, wherein performing an exclusive OR operation on the swap key and the first ciphertext is performed on the first swap key and the first block of the first ciphertext to produce a first block of the second ciphertext.

4. The method according to claim 3, wherein the first ciphertext comprises n blocks, and wherein for each of the second to nth blocks of the first ciphertext, the method further comprises: incrementing the counter value; and repeating the method of claim 3 using the incremented counter value.

5. The method according to claim 3, wherein the mode of operation is Galois/counter mode, and wherein the method further comprises: receiving a first authentication tag associated with the first ciphertext; receiving a second authentication tag associated with the second ciphertext; computing a third authentication tag using a Galois message authentication code function with the first ciphertext as the key; computing a fourth authentication tag using a Galois message authentication code function with the second ciphertext as the key; comparing the first and third authentication tags; and comparing the second and fourth authentication tags.

6. The method according to claim 5, wherein computing the third authentication tag comprises: i) performing Galois field multiplication on first authentication data to produce a first authentication stream; ii) performing an exclusive OR operation on the first authentication stream and the first block of the first ciphertext; iii) performing Galois field multiplication on the product of step ii; iv) repeating steps ii and iii for every block of the first ciphertext; v) performing Galois field multiplication on the product of step iv; vi) performing an exclusive or operation on the product of step v and a first length authentication vector, wherein the length authentication vector is a length of the first authentication data concatenated with a length of the first ciphertext; vii) performing Galois field multiplication on the product of step vi; and viii) performing an exclusive or operation on the product of step vii and the first encryption data.

7. The method according to claim 6, wherein computing the fourth authentication tag comprises performing the method of claim 6 on second authentication data, the second ciphertext, a second length authentication vector, and the second encryption data, wherein the second length authentication vector is a length of the second authentication data concatenated with a length of the second ciphertext.

8. The method according to claim 2, wherein for a first block of the first ciphertext, the method further comprises: encrypting a first initialisation vector with the first key to produce first encryption data; encrypting a second initialisation vector with the second key to produce second encryption data; and performing an exclusive OR operation on the first and second encryption data to produce a first swap key, wherein performing an exclusive OR operation on the swap key and the first ciphertext is performed on the first swap key and the first block of the first ciphertext to produce a first block of the second ciphertext.

9. The method according to claim 8, wherein the mode of operation is cipher feedback mode, wherein the first ciphertext comprises n blocks, wherein the second ciphertext comprises n blocks, and wherein for the nth block of the first ciphertext, the method further comprises: i) encrypting the (n−1)th block of the first ciphertext; ii) encrypting the (n−1)th block of the second ciphertext; and iii) performing an exclusive OR operation on the product of step i and the product of step ii to produce an nth swap key, wherein performing an exclusive OR operation on the swap key and the first ciphertext is performed on the nth swap key and the nth block of the first ciphertext to produce an nth block of the second ciphertext.

10. The method according to claim 8, wherein the mode of operation is output feedback mode, wherein the first ciphertext comprises n blocks and wherein the second ciphertext comprises n blocks, and wherein for the second block of the first ciphertext, the method further comprises: i) encrypting the first encryption data to produce second block first encryption data; ii) encrypting the second encryption data to produce second block second encryption data; and iii) performing an exclusive OR operation on the second block first encryption data and the second block first encryption data to produce a second swap key, wherein performing an exclusive OR operation on the swap key and the first ciphertext is performed on the second swap key and the second block of the first ciphertext to produce an second block of the second ciphertext.

11. The method according to claim 10, wherein for the nth block of the first ciphertext, the method further comprises: i) encrypting the (n−1)th block first encryption data to produce nth block first encryption data; ii) encrypting the (n−1)th block second encryption data to produce nth block second encryption data; and iii) performing an exclusive OR operation on the nth block first encryption data and the nth block second encryption data to produce an nth swap key, wherein performing an exclusive OR operation on the swap key and the first ciphertext is performed on the nth swap key and the nth block of the first ciphertext to produce an nth block of the second ciphertext.

12. A data processing apparatus for processing a first ciphertext using a block cipher, wherein the first ciphertext is data encrypted with a first key and not encrypted with a second key, the apparatus comprising: at least one processor configured to: receive a swap key, wherein the swap key is the product of an exclusive OR operation performed on the first key and the second key; receive the first ciphertext, wherein the first ciphertext comprises a plurality of blocks; and perform an exclusive OR operation on the swap key and the first ciphertext to produce second ciphertext, such that the second ciphertext is the data encrypted with the second key and not encrypted with the first key.

13. A non-transitory computer-readable storage medium for processing a first ciphertext using a block cipher, wherein the first ciphertext is data encrypted with a first key and not encrypted with a second key, wherein the non-transitory computer-readable storage medium comprises instructions which, when executed by a computer, cause the computer to carry out the steps of: receiving a swap key, wherein the swap key is the product of an exclusive OR operation performed on the first key and the second key; receiving the first ciphertext, wherein the first ciphertext comprises a plurality of blocks; and performing an exclusive OR operation on the swap key and the first ciphertext to produce second ciphertext, such that the second ciphertext is the data encrypted with the second key and not encrypted with the first key.

14. The method according to claim 4, wherein the mode of operation is Galois/counter mode, and wherein the method further comprises: receiving a first authentication tag associated with the first ciphertext; receiving a second authentication tag associated with the second ciphertext; computing a third authentication tag using a Galois message authentication code function with the first ciphertext as the key; computing a fourth authentication tag using a Galois message authentication code function with the second ciphertext as the key; comparing the first and third authentication tags; and comparing the second and fourth authentication tags.